There is a wireless sensor network (WSN) in which a number of nodes provided with a sensor are disposed and each node exchanges data by wireless communications, thereby collecting various types of measurement information from each node. For example, the WSN employs network structures, which include the tree type and the mesh type.
FIG. 22 is an explanatory view (1) illustrating an example of the tree type network structure. The tree type network structure, which provides a relatively easy parent-child relation management and motivation control, is suitable for those nodes that have only a weak processing capability. On the other hand, for the tree type network structure, only one node or a plurality of nodes may have a child node in each layer.
In the following descriptions, the node that has a child node will be expressed as a sink node, as appropriate.
In the example illustrated in FIG. 22, the network structure of a network 10 has one sink node in each layer. Of nodes 11 to 23 included in the network 10, the sink nodes having a child node are the nodes 11, 13, 18, and 22. Like the network 10, a network having one sink node in each layer is easy to manage as compared with a network having a plurality of sink nodes.
Here, in the tree type network structure, a failure of a sink node makes it impossible to communicate with all the nodes subordinate to the failed sink node. FIG. 23 is an explanatory view illustrating a problem of the tree type network structure. For example, in FIG. 23, a failure having occurred at the node 13 serving as a sink node disables the nodes 11, 12, 14, and 15 to communicate with the nodes 16 to 18 and nodes subordinate to the node 22 (not illustrated). Thus, the tree type network structure is low in robustness.
In contrast to this, for the mesh type network structure, each node is connected like a mesh, so that there exist a plurality of types of paths to a destination node. For this reason, even when a node has failed, it is possible to continue data communications by selecting a path that detours around the failed node, thus providing enhanced robustness.
On the other hand, even with the tree type network structure, robustness can be enhanced by setting the radio wave output from each node to a relatively higher level. FIG. 24 is an explanatory view (2) illustrating an example of the tree type network structure. The network structure of FIG. 24 is the same as that of FIG. 22. In the example illustrated in FIG. 24, it is assumed that the radio wave output from the node 11 is set to a relatively higher level. In this case, even when the node 13 serving as a sink node has failed, the radio wave from the node 11 reaches the node 18, and thus the nodes 11, 12, 14, and 15 can continue communications with subordinate nodes.    Patent Literature 1: Japanese Laid-open Patent Publication No. 2011-223419    Patent Literature 2: Japanese Laid-open Patent Publication No. 2011-205556
However, the aforementioned conventional technique does not enhance the robustness of networks without increases in costs during operation.
The aforementioned mesh type network structure can provide an enhanced robustness. However, the structure sends and receives packets for periodically updating paths, thus causing an increase in cost during operation. Furthermore, even the technique for increasing the level of radio wave output from each node may also cause an increase in cost during operation due to an increase in power consumption.